This invention relates to power transmission systems utilizing toothed power transmission belts cooperating with toothed pulleys, and also related to pulleys for use in such a system.
Power transmission systems are known utilizing belts having a plurality of alternating teeth and grooves extending generally transversely of the belt and meshing with alternating teeth and grooves in a grooved pulley in order to perform a driving function. Many proposals for such belts and pulleys have been made, and in particular considerable work has been done on designing the optimum shapes for the cross-sections for the teeth and grooves on both the belt and the pulley. For example, U.S. Pat. No. 3,756,091 to Miller discloses a positive drive system in which the belt includes relatively closely spaced together driving teeth of a special curvilinear cross-sectional configuration which is operated in conjunction with mating pulley grooves defined between pulley teeth having curvilinear cross-sections which are substantially conjugate to the teeth of the belt. Given a toothed belt in combination with a pulley, the conjugate form of the belt tooth is that tooth form described on the pulley which corresponds to the volume between the belt and the pulley which is not swept out by the belt tooth as the belt moves into contact with the pulley. For any given belt tooth, the conjugate form of pulley tooth can readily be determined graphically. Use of conjugate teeth on the pulley, desirably with small deviations from a true conjugate by the removal of additional material in the tip area of the tooth, is desirable as it ensures that the belt and pulley teeth are able to engage and disengage without interference.
For the purposes of this specification a groove formed between adjacent conjugate teeth will hereinafter be referred to as a conjugate groove. As used herein the terms describing the features of the present invention are defined in the patent to Miller.
U.S. Pat. No. 2,507,852, to R. Y. Case, describes a power transmission belt comprising an inextensible tensile member having teeth bonded to one side and a protective jacket fabric covering the teeth. The teeth are preferably made of an elastomeric material, such as rubber, and the belt may also include a backing layer of identical or similar material to that which the teeth are constructed.
Many different elastomeric materials have been utilized for the construction of belts made in accordance with the Case patent, some of the more common materials being neoprene and polyurethane. These belts are designed to mesh with toothed pulleys that are constructed of a material having a higher Young's modulus than the elastomeric material used for the construction of the belt. The conventional toothed belt, as described in the Case patent, utilizes a tooth cross-sectional configuration that is essentially trapezoidal and which is very similar to a conventional rack tooth. Many attempts have been made to alter the belt and pulley teeth configurations to relieve the problem of belt failure. In such trapezoidal tooth belts, the common failure is that of tooth shear due to stress concentration. In seeking to reduce tooth shear, U.S. Pat. No. 3,756,091 to H. Miller, discloses belt teeth having a cross-sectional configuration which approximates the contour of the one-half order isochromatic fringe in a belt tooth under a defined rated load. The pulley grooves in accordance with the Miller patent are in mating engagement with and are substantially conjugate to the belt teeth. The belt having substantially curvilinear teeth in accordance with the Miller patent reduced belt tooth shear and increased horsepower capacity. One mode of failure in the belt configuration according to Miller may occur because of land wear in the belt, especially with small diameter pulleys. Land wear between the belt teeth is due to abrasion of the proective layer and exposure of the tensile member by action of the pulley tooth against the belt. This land wear leads to premature failure due to a detachment of the teeth from the tensile member and/or a break in the tensile member.
U.S. Pat. No. 4,037,485, to Hoback, proposes a solution to the land wear problem. As disclosed in the Hoback patent, the dimensional relationship of the belt teeth and grooves and the pulley teeth and grooves is such that in the longitudinal extent of the belt between the pulleys, the height of the belt teeth is greater than the depth of the pulley grooves while as the belt travels around the pulleys, the extreme outwardly facing portions of the belt teeth which confront the pulleys comes into contact with the portions of the toothed pulley which define the bottom of the pulley grooves. At the same time, Hoback discloses that the belt teeth are compressed to reduce their height so that the extreme radially outwardly facing portions of the pulley teeth come into contact with the portions of the belt disposed between the belt teeth which define the bottoms of the belt grooves.
Relatively early in the operating life of the toothed power transmission belt and pulley as disclosed in Hoback, the compressive engagement of the elastomeric teeth against the bottom of the pulley groove results in a substantially permanent deformation of the belt tooth. This deformation is accelerated at elevated operating temperatures such as occur in automotive applications. The deformation results in a significant permanent decrease in the tooth height. The deformed teeth no longer support the tensile member as desired by Hoback. Subsequently, the same wear pattern develops in the toothed belt and pulley drive, according to Hoback, as in the toothed power transmission configuration according to the Miller patent.
Belt and pulley combinations as disclosed in the Miller U.S. patent are now in general use and have been found to give excellent results where the toothed pulley has, for example, 30 grooves or more and where such grooves are on an 8 mm pitch. In practice, this limitation imposes a minimum pulley diameter for any given tooth pitch and size combination, below which diameter belt life could be adversely affected depending on operating loads and conditions.